1. Technical Field
This invention relates to a method and apparatus for data logging.
2. Related Art
Data logging in this specification is the process of collecting data from mobile devices performed in order to obtain business information relating to how the mobile devices operate. For example, position, location and speed of a vehicle over time is useful log data for use in insurance liability calculations for that vehicle. In another example, signal strength of a mobile communication system over both time and position is useful log data to enable a telecommunication company to plan its transmitter locations. Such data is collected by a mobile embedded system using positional information and signal strength information from sources including the network itself (e.g. GSM) and global positioning satellites (GPS). Log data is stored in the mobile embedded system for later transmission to the central system. Transmission is by mobile phone network or other wireless technology.
Transmission of the data log may be performed on demand; when the device is ready it requests control of the transmission channel. Such a system is described in US Patent Publication 6263268 which discloses a mobile automotive telemetry system for installation on-board a vehicle. It includes a diagnostic structure for monitoring operational functions of a vehicle and a server which communicates with the diagnostic structure to receive operational information. The operational information is uploaded to the server when the information is ready.
Another download on demand system, International Patent Publication 02/03350, discloses a method and system for monitoring cellular communication. The method continuously extracts traffic load and speed on roads within the coverage area of a cellular network from a mobile device in a vehicle. The data is extracted directly from the higher level of communication in a cellular network so there is no scheduled or negotiated download of data from the mobile device.
One problem with downloading log data on demand is that it can lead to a conflict situation when several devices are requesting control of a single channel and attempting to download at the same time. Only one request per channel will be successful at any one time and the other requests fail. The failed requests use download resource so that more resource is used for non-ordered requests than for ordered requests. One way to order the downloads is to schedule them to come in at a certain times.
US Patent publication 2001/0028313 discloses a distributed telemetry method and system affected by coordinating the taking of readings of a parameter by mobile phone users, the parameter readings being sent to a service system together with location information on the users. It is the task of a query scheduler to, amongst other things, organise when the reading of interest are to be taken. The reading is sent to the service system immediately or triggered by, for example, a scheduled time.
The problem with scheduled remote data logging is that simultaneous and multiple device upload of data can create overload on a server that collects such log data when the download size is different from that scheduled.